An analysis of environmental and economic implications of nil and restricted grazing systems designed to reduce nitrate leaching from New Zealand dairy farms. I. Nitrogen losses

Nitrate leaching is perceived to be a serious consequence of dairy farming due to the uneven return of N in small concentrated urine patches. Management systems in which the direct deposition of urine is avoided throughout the year (nil grazing) or during autumn/winter when the risk of nitrate leaching is highest (restricted grazing) could potentially reduce nitrate leaching. However, possible disadvantages of such systems include a reduction in the clover content of pastures, increases in gaseous losses, and increases in capital and/or operating costs. This paper examines some of the effects of nil and restricted grazing management systems for dairy farming on N flows and losses to the environment. The estimates of N losses are based on the results of a long-term farmlet study under conventional grazing, on data for an average New Zealand farm, and on literature information. The analysis showed that in nil grazing systems nitrate leaching losses may be reduced by 55-65% compared with conventional grazing systems, and by 35-50% in restricted grazing systems. For nil grazing systems, however, total N losses were 10-35% higher than under conventional grazing because of increased gaseous losses. The total N losses from restricted grazing systems were similar (-10 to +5%) to those from conventional systems. The analysis showed the potential benefit of a restricted grazing system as a management tool to reduce nitrate leaching losses, especially in areas where contamination of ground and surface waters is of particular concern.